It is known that a silica-based optical fiber whose core region is doped with an appropriate luminescent (typically rare earth) species can serve as optical amplifier for signal radiation of an appropriate wavelength, or as a coherent radiation source, a laser. See, for instance, U.S. Pat. Nos. 4,923,279 and 5,058,976, both incorporated herein by reference.
The '279 patent teaches that it is advantageous to provide a fiber with the highest concentration of the luminescent species (e.g., Er) at the center of the core, since such a distribution results in improved gain performance for a given pump power.
The '976 patent discloses Er-doped fiber designs which can provide fiber having, inter alia, low amplification threshold and noise. Methods of making such fiber are also disclosed. See also B. Pedersen et al., Journal of Lightwave Technology, Vol. 9, p. 1105 (1991), which suggests fiber designs for single mode operation.
Although optical fiber amplifiers in tests have already attained quite high performance levels, currently known designs do not yet optimally satisfy all requirements of practical optical fiber communication systems. For instance, currently known designs for 1.48 .mu.m pumped amplifiers are relatively sensitive to variations in cut-off wavelength of the fiber. This is disadvantageous, since it is typically difficult to control the cut-off wavelength of single mode optical fiber to within better than .+-.5%. It is particularly difficult to control the cut-off wavelength to better than about .+-.5% in the small core, highly doped fibers typically used as optical amplifiers. As a further example, currently known designs for 0.98 .mu.m pumped amplifiers are relatively sensitive to the distribution of the pump power among the spatial modes of the (single mode for the 1.5 .mu.m signal radiation) fiber. For instance, the effect of launching the higher-order LP.sub.11 mode has been investigated by J. R. Armitage, Applied Optics, Vol. 27, p. 4831 (1991). For fibers with an LP.sub.11 cut-off wavelength .lambda..sub.c &gt;980 .mu.m, Desurvire et al. [(Journal of Lightwave Technology, Vol. 8, p. 1730 (1990)] have demonstrated a variation of as much as 20 dB in amplifier gain when the pump power (.lambda..sub.p =0.98 .mu.m) shifts from the LP.sub.01 to the LP.sub.11 mode in a fiber with .epsilon.=0.25, (.epsilon.=r.sub.Er /a, where r.sub.Er is the effective radius of the Er-doped inner core region, and a is the effective radius of the core. See also FIG. 1).
In view of the great commercial significance of optical waveguide amplifiers and lasers, it would be highly desirable to have available a waveguide design which overcomes (or is at least less subject to) the above discussed shortcomings. This application discloses such a design, and waveguides (including silica-based Er-doped optical fibers and planar waveguides) that embody the inventive design.